1. Field of the Invention.
The invention relates to radio systems and particularly to non-transparent data transmission in a mobile communication system where a mobile services switching centre and a radio access network belong to different system generations.
2. Description of Related Art
Mobile communication systems generally refer to different telecommunication systems which enable personal wireless data transmission while subscribers roam in the system area. A typical mobile communication system is a Public Land Mobile Network (PLMN). First-generation mobile communication systems were analog systems where speech or data was transferred in an analog form similarly as in conventional public switched telephone networks. An example of a first-generation system is the Nordic Mobile Telephone (NMT).
In second-generation mobile systems, such as the Global System for Mobile Communication (GSM), speech and data are transmitted in a digital form. In addition to conventional speech transmission, digital mobile communication systems provide a plurality of other services: short messages, facsimile, data transmission, etc. Services provided by mobile communication systems can generally be divided into teleservices and bearer services. A bearer service is a telecommunication service which provides signal transmission between user-network interfaces. For example modem services are bearer services. In a teleservice the network also provides subscriber terminal services. Important teleservices include speech, facsimile and videotex services. Bearer services are usually divided into groups according to a property, such as asynchronous and synchronous bearer services. Each of these groups comprises a number of bearer services, such as a transparent service (T) and a non-transparent service (NT). In a transparent service the data to be transmitted is unstructured and transmission errors are corrected only by means of channel coding. In a non-transparent service the data to be transmitted is structured into protocol data units (PDU) and transmission errors are corrected by utilizing (in addition to channel coding) automatic retransmission protocols. For example in the GSM system such a link protocol is called a radio link protocol (RLP). This kind of link protocol is also generally referred to as link access control (LAC).
Currently under development are third-generation mobile communication systems, such as the Universal Mobile Communication System (UMTS) and the Future Public Land Mobile Telecommunication System (FPLMTS), which was later renamed as the International Mobile Telecommunication 2000 (IMT-2000). The UMTS is being standardized by the European Telecommunication Standards Institute (ETSI), whereas the International Telecommunication Union (ITU) standardizes the IMT-2000 system. These future systems are basically very similar. For example the UMTS, as all mobile communication systems, provides wireless data transmission services to mobile subscribers. The system supports roaming, which means that UMTS users can be reached and they can make calls anywhere as long as they are situated within the coverage area of the UMTS.
According to the current view, a UMTS consists of two or three parts, which are illustrated in FIG. 1: a UMTS access network 1 (or a UMTS base station system UMTS-BSS) and a core network 2, 3, 4 and 5. The UMTS access network will be referred to below generally as a radio access network. The UMTS access network 1 is mainly responsible for matters related to the radio path, i.e. it provides the core network with radio access required for wireless operation. The core network 2, 3, 4 or 5 is a conventional or future telecommunication network, which has been modified to utilize the UMTS access network efficiently in wireless communication. Telecommunication networks that are applicable as core networks include second-generation mobile communication systems, such as the GSM (Global System for Mobile Communication), ISDN (Integrated Services Digital Network), B-ISDN (Broadband Integrated Services Digital Network), packet data networks PDN, ATM (Asynchronous Transfer Mode), etc.
Therefore a UMTS access network should enable support for different core networks including networks that will possibly be developed in the future. Correspondingly, UMTS access networks should enable the connecting of different radio interfaces to the core network (narrowband, broadband, CDMA, TDMA, etc.). According to the present scenario the functions of a UMTS access network are strictly limited to radio access functions. Therefore the network mainly comprises functions for controlling radio resources (handover, paging) and for controlling bearer services (radio network service control). The more complicated functions, such as registers, register functions, mobility management and location management, are placed in each core network or in service providers which provide UMTS subscribes with different services and are connected to the core network.
According to UMTS terminology, the entire UMTS access network is called a generic radio access network (GRAN). The GRAN is further divided into a radio access network (RAN) and an interworking unit (IWU). In principle, between each core network 2-5 and RAN there is a separate IWU, such as IWUs 1 to 4 shown in the figure. The purpose of the IWU is to provide the connection between the core network and the RAN. Therefore the IWU comprises the required adaptations and other possible interworking functions. The interface between the IWU and the CN is specific to the core network. This enables the development of the core networks and the RAN independently of one another. For example, the IWU may be connected to a base station system BSS in a GSM network. Correspondingly, IWU2 may be connected to a local exchange in an ISDN network, for instance. FIG. 1 also shows service providers SP2, SP3, SP4 and SP5, which are connected to core network CN2.
In FIG. 1 a radio access network RAN comprises a transport network TN, a radio network controller RNC and a base station BS. In the network architecture shown therein, the base stations are connected to the TN, which conveys the user data to the IWUs and the control signalling to the RNC. All the intelligence controlling the GRAN is placed in the base stations BS and in the radio network controller RNC. As stated above, this control is typically limited to control functions related to the radio access as well as to the switching of connections through the transport network. The TN may be, for example, an ATM network. However, it should be noted that only one possible implementation of a UMTS access network is described above.
Transition to the use of third-generation mobile communication systems will take place gradually. At the beginning, third-generation radio access networks will be used in connection with network infrastructure of second-generation mobile communication systems. Such a hybrid system is illustrated in FIG. 2. A second-generation mobile services switching centre MSC is connected both to a second-generation radio access network, such as a GSM base station system BSS consisting of a base station controller BSC and base stations BTS, and to a third-generation radio access network consisting of, for example, a radio network controller RNC, an interworking unit IWU and base stations BS. According to a scenario of the GSM MSC, between the MSC and the third-generation radio access network there is preferably a standard A interface. The IWU performs a physical layer conversion, such as a conversion from the ATM to the Primary Rate (E1/T1) and vice versa, and a protocol level conversion, for example a conversion from third-generation rate adaptation and link access protocol (called hereinafter a link access control protocol LAC) to GSM rate adaptation and an L2R/RLP protocol and vice versa in non-transparent transmission, and a signalling conversion, for example from third-generation signalling to GSM A-interface signalling and vice versa. By means of this configuration (standard A interface) a third-generation access network can be connected to any existing second-generation MSC without any modifications in the MSC. This also ensures inter-manufacturer compatibility, i.e. an RNC/IWU of one manufacturer is compatible with an MSC of another manufacturer.
In practice, there will be two different radio subsystems RSS, which share a common infrastructure on the network subsystem NSS level. Second-generation mobile stations MS (such as the GSM) communicate via the second-generation radio access network and third-generation mobile stations MS (such as the UMTS) communicate via the third-generation radio access network. Possible dual-band phones (such as GSM/UMTS) are able to use either radio access network and to perform handovers between them.
However, one problem is related to this network configuration in an inter-RNC handover between radio access networks. If an inter-RNC handover takes place, the RNC/IWU changes, wherefore also the third-generation link protocol unit (situated in the RNC/IWU) changes. This means that the contents of the data buffers in the RNC/IWU are lost. It is possible to restore them only at the application level (between end users). This is contrary to the principles of non-transparent transmission where data integrity should be maintained during transmission. In practice it means that the network configuration disclosed above does not support an inter-RNC handover.
Therefore an object of the invention is to develop a data transmission method and a network configuration maintaining data integrity in a handover for a non-transparent call between two radio access networks.
The invention relates to a mobile communication system, an interworking unit of a radio access network, a mobile services switching centre, and a call control method.
The basic idea of the invention is to provide a second-generation mobile services switching centre with a protocol unit that also supports a radio link protocol of a third-generation radio access network. In such a case a radio link protocol is set up between a mobile station and a mobile services switching centre without a radio link protocol conversion in the interworking unit of the radio access network. The radio access network merely forwards the radio link protocol transparently between the MS and the MSC, i.e. it extends the protocol to the MSC. When a non-transparent call is subjected to a handover between radio access networks, the same radio link protocol units (in the MS and the MSC) are also used after the handover. Possibly ongoing sequences of selective retransmissions and retransmission requests of the radio link protocol are not interrupted or disturbed, wherefore it is possible to also avoid the manipulation of buffer synchronization which might lead to retransmission complications and the loss or doubling of data as a result of the handover.
In order that the changes in the MSC would be as slight as possible, it is preferable to keep the interface between the MSC and the third-generation radio access network similar to the interface between the MSC and the second-generation radio access network, except for the radio link protocol. In such a case changes in the MSC can be restricted at a minimum to the implementation of the interworking function. However, the third-generation radio access network and the interface between the radio access network and the MSC comprise different rate adaptations and/or physical channels and/or signalling forms. In an embodiment of the invention, an interworking unit of a third-generation radio access network is arranged to carry out a conversion or adaptation between the aforementioned rate adaptations and/or physical channels and/or signalling types.
However, in practice it may be difficult to arrange all the second-generation MSCs to also support a third-generation radio link protocol. Similarly, all third-generation radio access networks will not necessarily enable the transmission of a radio link protocol transparently to the MSC. This might lead to problems with compatibility between the products of different manufacturers.
According to a preferred embodiment of the invention, an interworking unit in a third-generation radio access network supports both conventional second-generation mobile services switching centres and mobile services switching centres according to the invention with two protocols. In the first operating state the interworking unit does not carry out a radio link protocol conversion but it relays the radio link protocol transparently between the mobile station and the mobile services switching centre in both directions. In the second operating state the interworking unit carries out a protocol conversion between the radio link protocol supported by the radio access network and the radio link protocol supported by the mobile services switching centre. The interworking unit uses the arrangement that is supported by the mobile services switching centre connected thereto. If the mobile services switching centre supports both the second-generation and the third-generation radio link protocol, the interworking unit employs the first operating state in order to ensure successful handovers between radio access networks. If the mobile services switching centre only supports the second-generation radio link protocol, the interworking unit employs the second operating state which ensures compatibility but not successful handovers between radio access networks. The operating state may be set fixedly for example in connection with installation. In such a case changing the operating state later requires a separate maintenance or installation procedure. Alternatively, the interworking unit may select dynamically, specifically for each call, the operating state according to the properties of the mobile services switching centre. In such a case the interworking unit automatically uses the correct protocol and no procedures are required by the maintenance personnel. Dynamic selection can be based, for example, on data, a parameter or a command contained in outband signalling of the mobile services switching centre. The selection can also be based on identification of the protocol used by the other party from inband communication.
According to the preferred embodiment of the invention, a mobile services switching centre which supports both a second-generation and a third-generation radio link protocol selects the protocol it uses dynamically, specifically for each call, according to the radio link protocol that is supported by the radio access network through which the call will be switched. If the radio access network supports both the second-generation and the third-generation radio link protocol also at the interface between the mobile services switching centre and the radio access network, the MSC selects the third-generation protocol in order to ensure successful handovers between radio access networks. If the radio access network only supports the second-generation radio link protocol at the interface between the mobile services switching centre and the radio access network, the MSC selects the second-generation radio link protocol in order to ensure compatibility. Dynamic selection can be based, for example, on data, a parameter or a request contained in outband signalling of the radio access network. The selection can also be based on identification of the protocol used by the other party from inband communication. Further, the selection can be based on prior data the mobile services switching centre has concerning the network configuration, i.e. which radio access network supports which protocol.